All of the above patent applications are hereby incorporated herein by reference.
Provisional Application No.:
No. 60/010,726 filed Jan. 29, 1996
References Cited:
U.S. Pat. Nos. 5,578,526, 5,502,289, 5,495,398, 5,495,394, 5,436,411, 5,426,566, 5,422,435, 5,324,952, 4,833,334
U.S. patent applications Ser. Nos. 08/221,506, 08/372,289
The present invention relates in general to an improved method for radiation shielding of microelectronic devices. The invention more particularly relates to packaging designs and processes for manufacturing improved radiation tolerant 3-dimensional ceramic and metal packaged microelectronic multi-chip modules (MCMs).
Many of today""s commercial integrated circuit (IC) devices and multi-chip modules (MCM) cannot be utilized in deep space and earth orbiting applications because of Total Dose radiation induced damage. Commercial IC devices are developed and manufactured for the computer and mass market applications and are not designed to withstand the effects of the natural space environment. The type and source of radiation in space include solar flares, galactic cosmic radiation and the Van Allen trapped electron and proton belts or man-made radiation induced events (neutrons and gamma radiation).
Commercially available metal packaged integrated circuits and multi-chip modules have not been used in spacecraft applications because of both perceived and real reliability problems. Some examples of commercial multi-chip modules are U.S. Pat. Nos. 5,578,526, 5,502,289, 5,495,398, 5,495,394, 5,436,411, 5,422,435. The major issues needing to be addressed for commercial integrated circuits in order for them to fly in space are the reliability and survivability of these devices when exposed to spacecraft environmental hazards such as total dose levels of electrons, protons, solar flares, and cosmic radiation. Typical silicon integrated circuit plastic, ceramic, metal and multi-chip module packaged devices will fail to operate when exposed to total doses of 2 to 15 Krads(Si). Since communication satellites are expected to function in orbit for periods of 8 to 15 years, this would rule out almost all commercially available packaged silicon integrated circuit devices and multi-chip modules.
Common methods used to prevent radiation degradation in performance for integrated circuits are: 1) design special radiation tolerant die, 2) screen each part for radiation tolerance, or 3) shield the package or the platform. There are tradeoffs with each of these methods. The first example usually is the most radiation tolerant. Here the die is specially designed to be radiation tolerant. However, this method is both time consuming and expensive to produce since the part must be redesigned to incorporate radiation hardening techniques. Examples of this method include U.S. Pat. Nos. 5,324,952, 5,220,192, 5,140,390, 5,024,965, 5006,479, 5,001,528, 4,903,108, 4,825,278, 4,675,978, 4,402,002, 4,313,7684, 4,148,049, 4,014,772, and 3,933,530. This method delays the time to market such that these radiation hardened devices are usually 2 to 3 generations behind the current commercial technological advances in both size and capabilities. There are additional penalties in limited marketability and demand for the product. The result is a higher cost from low volume productions of the die. The end result, is that this method produces; 1) a more expensive product that is 2) technologically behind current commercially available microelectronics, 3) frequently with slower speed and 4) less capability. Additionally because of the limited market for these products, frequently they are not available at all.
The second method involves testing each part or die lot in the hopes that the die will meet the mission radiation requirements. This could be an expensive process because of the large amount of testing that would be required and the low probability of success in finding an inherently radiation tolerant die that meets the mission requirements. This problem is compounded for Multi-chip Modules in that all the die required in the package need to be radiation tolerant. This is not only extremely restrictive on the design and expensive because of the amount of testing but it is highly unlikely that all the required die will be found that will meet the radiation requirements of the mission.
The third method involves shielding the part. This method includes either shielding the entire satellite, subsystem or individual part. Shielding the satellite or subsystem carries extreme weight and size penalties that generally make this solution cost prohibitive. The spacecraft has some inherent shielding the skin and spacecraft components, however this is very difficult to model and generally doesn""t provide adequate shielding for all parts and directions.
An example of system level shielding is U.S. Pat. No. 4,833,334, which is incorporated by reference as if fully set forth herein, describes the use of a protective box to house sensitive electronic components. This box is partially composed of a high atomic weight material to effectively shield against x-rays. However this approach has the serious disadvantage off adding substantial bulk and weight to electronic circuit assemblies protected in this manner. Moreover, it would be expensive to provide this type of protection to individual integrated circuits as manufacturing custom boxes for each circuit configuration would be costly. Similarly U.S. Pat. No. 5,324,952, follows a method of shielding components. If shielding is required the better method is to shield only the components that require shielding.
One method of shielding individual components is know as spot shielding. With this method, a small shield is attached to the surface of the package. However this method does not provide effective 3-dimensional shielding protection. Additionally, the external shield is generally thermally mismatched to the package, and increases the size and weight of the package. Often a bottom spot shield cannot be used due to the inability to accommodate a fixed lead length. The spot shield also has no mechanical support except the adhesive used to attach it to the surface of the package.
An example of spot shielding is disclosed in Japanese patent publication 62-125651, published Jun. 6, 1987 which is incorporated by reference as if fully set forth herein. This patent describes a spot shielded semiconductor device which utilizes a double layered shield film to a sealing cover on an upper surface of the semiconductor package and attaching another double layered shield film to a lower surface of the package. However, space qualified microelectronic parts must be capable of withstanding the enormous forces exerted during acceleration periods. The external shields are subject to tearing or prying off from the sealing cover. The use of a double layer shield film only slightly reduces the weight of the package but increases the size of the package unnecessarily. Also thin films are generally only effective at shielding Electromagnetic Interference (EMI) radiation and are ineffective at shielding ionizing radiation found in space. Examples of this type of EMI or EMF shielding devices include U.S. Pat. Nos. 4,823,523, 4,868,716 and 4,266,239.
The significant disadvantage of the spot shielding method include an increase in weight and thickness of the device, an increase in exposure of the semiconductor to side angle radiation due to the shielding being spaced apart from the semiconductor.
A better method of shielding involves using an integrated shield, where the package itself is the shield. The best example of this is Space Electronics Inc.""s RAD-PAK(copyright) technology, patent application Ser. No. 08/372,289 where the material in the package and the package design is optimized for the natural space radiation environment. However this method focuses on single-sided MCMs and monolithic ICS. These designs are acceptable for most applications but do not maximize the density of integrated circuit designs.
The inventions described herein will provide:
Improved shielding in all axial directions
Ability to take advantage of current generation IC technological advances
Lower cost due to
The use of commercially available dies at market prices
Improved Delivery times
Higher density of integrated circuits
In addition, the inventions are improvements to patent application Ser. No. 08/372,289, titled Radiation Shielding of Integrated Circuits and Multi-Chip Modules in Ceramic and Metal Packages. These designs provide 3-dimensional techniques which result in lighter and more dense Multi-Chip modules (MCMs). Several new design approaches are described, each with its attendant advantages and cost/performance characteristics.
A typical prior art metal or ceramic packaged integrated circuit or multi-chip module assembly consists of silicon integrated circuit die mounted on a substrate (ceramic) which is then mounted to the metal base with wirebonds connecting the substrate to the wire bond package pads/posts. (FIG. 1) The base is sealed with a metal lid using resistance welding or solder sealing techniques.
The final packaged devices are tested for conformance to manufacturer""s specifications and those that pass are delivered. These devices would not work in the typical space application xe2x80x9cas isxe2x80x9d since the housing is very thin (approximately 3 to 8 mils/metal or approximately 10 to 40 mils/ceramic) and is designed for mechanical structures only.
A ceramic or metal (usually Kovar) lid is utilized to seal the package. Typical packages do not use metal on the base except for heat sinking purposes and metallization of the ceramic for wire bond and die attach purposes.
The process begins with use of commercially available software like xe2x80x9cSpace Radiation Version 4.0xe2x80x9d to model the application environment based on orbit or trajectory information. A dose versus depth curve is the generated output. With the dose versus depth curve and knowing the radiation tolerance of the dice, the required amount of shielding to be used in the package can be calculated. By plotting the die tolerance on the dose versus depth curve, and the inherent satellite shielding, the required amount of shielding from the integrated circuit package can be determine to insure that the integrated circuit will survive over the satellite mission life.
In the invention, the 3-dimensional radiation shielded MCM is comprised of a double-sided substrate (either ceramic or printed circuit board) with IC die mounted to both sides. Two configurations of substrates exist, substrates mounted into packages and substrates which are an integrated part of the package. In all configurations the packages must be hermetically sealed if the part is to be used in space environments.
The first configuration involves attaching the substrate within a base package comprised of radiation shielding material. Electrical connections are made from the substrate through insulating feed throughs that are attached to package leads. A radiation shielding lid is sealed to the base to from a hermetic seal.
The second configuration integrates the substrates into the package. This configuration consists of; a double-sided substrate (ceramic or printed circuit board) with the IC die attached and wire bonded, a lid and side-wall combination comprised of radiation shielding material which is sealed to the substrate with a seal ring. Electrical connections are made from the screened inter connects within the substrate which are then attached to the external package leads. (FIG. 4)
To mechanically hold the substrates in the third configuration, the substrates is sealed inside a package comprised of side walls and lids comprised of radiation shielding material or sandwiched between the side-walls. The package is designed with two cavities, one for each substrate or a single integrated top and bottom sided substrate. The substrates are wire bonded to the package wire bond pads The wire bond pads connect to the external leads (FIG. 5).
In a fourth configuration, two substrates are mounted on a dual cavitied base (FIG. 11). The integrated circuits are mounted on the substrate with electrical connections made to package leads outside the package through insulating electrical feed throughs in the base. The two lids and the base are comprised of radiation shielding material.
In the third and fourth configuration, two lid types can be utilized per side: 1) A single shield which also is the lid and provides a hermetic seal (FIG. 6), or 2) two lids, an internal shield (lid) which does not provide a hermetic seal, and an outer xe2x80x9cstandardxe2x80x9d lid which provides the hermetic seal (FIG. 7). All lid types can be flat or provide sidewall shielding. This version is used when extra radiation shielding is required and for sealing and mechanical reasons, a non-radiation shielding material needs to be used to make a hermetic seal
The preferred embodiment of the invention for the 3-dimensional radiation shielded MCM concept will result in at least a ten fold improvement in the devices"" ability to meet a given total dose and is structurally and thermally stable. In the preferred embodiment shielding would be composed of a high Z material or a mixture of high Z/Low Z material. Where High Z is defined as material with an atomic number greater than 40.
Lead configurations can be of many types (pin-grid array, flat package, dual-in-line packages, can packages, etc.). The process and design steps to achieving a fully integrated, shielded device are described in the Preferred Embodiment.